Firetube boilers are commonly used to heat water for a variety of purposes. Firetube boilers typically are formed by a cylindrical vessel capped at each end. A plurality of parallel or substantially parallel heating tubes are disposed within the vessel. A main furnace tube is also defined within the vessel. A stream of high temperature gases is directed down the furnace tube from a main flame source. Upon reaching the opposite end of the vessel, i.e., the turnaround chamber, the heated gases are reflected through the parallel heating tubes of the firetube boiler. The number of times that these heated gases are reflected defines the number of "passes" employed by the boiler. Thus, if the stream of heated gases is reflected once, the boiler is considered to be a two-pass boiler. If the stream of heated gases is reflected twice, the boiler is considered to be a three-pass boiler.
In order to effect the desired magnitude of heating of the liquid within the boiler, very high levels of heat are directed through the furnace tube and the heating tubes of the boiler. As a result, it has been found that the internal surfaces of the turnaround chamber, in particular the rear wall of the turnaround chamber, are subjected to great erosion from the heated gases. In order to minimize the detrimental effects of the heated gases on the turnaround chamber, two alternative approaches have been developed.
Some boilers employ a ceramic refractory lining within the turnaround chamber in order to minimize erosion. Such boilers are sometimes referred to as "dryback" boilers. Boilers employing such a refractory lining have proven to be somewhat successful in reducing erosion of the turnaround chamber.
In some dryback boilers, the refractory-lined wall of the turnaround chamber is a door which permits access to the interior of the boiler, thereby facilitating service of the interior of the boiler. It has been found, however, that there is a significant level of heat loss realized through the ceramic refractory material due to its relatively poor insulating characteristics. In addition, the ceramic refractory material is also subject to the erosive effects of the hot gases over extended periods of use, thereby requiring replacement or repair of the door.
An alternative approach to resolving the erosion problems has given rise to what is known as a "wetback boiler." Rather than employing a refractory material to limit erosion of the turnaround chamber, the wetback boiler utilizes a body of water contained within the boiler to cool the walls of the turnaround chamber. The body of water "surrounds" the turnaround chamber on three sides, and thus essentially "submerges" the turnaround chamber. The presence of water around the turnaround chamber provides greater insulation than the refractory material discussed above, thereby minimizing heat loss from the boiler.
Although providing better resistance to erosion and better insulation characteristics as compared to the "dryback" firetube boiler, there are certain disadvantages associated with the wetback boiler. The wetback design often significantly reduces access to the interior of the boiler through the rear wall of the turnaround chamber. Although most wetback boilers include a small porthole through the cooling chamber, these portholes are typically very small in dimension, thereby reducing access to the interior of the boiler. Thus, it has been found to be considerably more difficult to service the interior of a wetback boiler as compared to dryback boilers having a larger access door disposed at the rear wall of the turnaround chamber.